The need for increased capacity accompanying the expansion of the wireless communications market is forcing a move away from analog modulation techniques, such as frequency modulation (FM), to digital modulation formats, such as time division multiple access (TDMA) and code division multiple access (CDMA). Since both TDMA and CDMA modulation require greater linearity than can be routinely obtained in an uncorrected, high efficiency (class AB) RF power amplifier, it is necessary to incorporate some form of amplifier distortion correction mechanism into the signal flow path through the amplifier. In addition, market forces are demanding amplifiers to simultaneously handle a multitude of narrow-band signals, thus also increasing the need for linearity.
The distortion introduced by the amplifier causes the phase and amplitude of the output signal to depart from the respective phase and amplitude of the input signal, and may be considered as an incidental (and undesired) amplifier-sourced modulation of the input signal. Observation and analysis of this distortion-introducing behavior of an RF power amplifier by the inventors has led to the conclusion that the distortion is primarily driven by the envelope (instantaneous amplitude) of the input signal. For example, as the amplitude of the input signal varies, the shape of the amplifier output signal does not exactly track that of the input signal. Also, the harder the amplifier is driven (by the peak of the signal), the larger the phase delay through the amplifier.